This invention relates to the field of compatible AM stereo broadcasting and more particularly to a system for differentiating between stereo and monophonic transmission.
In commercial broadcasting, whether AM or FM, it is likely that at any given time, some stations will be broadcasting stereophonic sound and others, monophonic. Also, any one station may, for example, broadcast musical programs stereophonically but "talk shows" or commercials monaurally. Most radio listeners prefer to know the broadcasting mode of a chosen station. More importantly, there are advantages in switching the receiver circuitry to a monophonic mode of operation when no stereo is being received.
In frequency modulated broadcasting, a 19 KHz pilot carrier used for regeneration of the suppressed carrier can also be used to enable a "stereo" indicator, typically a small lamp or LED display. The same signal is used to switch the receiver output circuitry from the monaural mode to a stereophonic mode. In the AM band, however, there is no available channel space for such an additional carrier and any signal used for indicating the presence of stereo or mode switching must be on the main carrier, within a 10 KHz channel assignment.